The present invention is directed to the enhancement of air/air packet data communications reliability among aircraft, and the enhancement of air/ground packet data communications reliability between aircraft and ground systems.
Aircraft commonly transmit and receive analog voice radio communications, to enable air traffic control and provide for other air traffic services (ATS), via radio equipment operating on selected frequency channels. A single aircraft typically supports at least one radio, and may support several radios, each radio tuned to a different frequency channel. Analog voice communications in the VHF band use frequency channels separated by 25 kHz between channel centers, and also 8.33 kHz between channel centers. The aviation industry is currently developing a digital voice and data capability for ATS which will also operate on 25 kHz channels in the VHF band.
Commercial aircraft may additionally support analog and digital air/ground communications for airline operational control (AOC). One example used in the VHF band is the Aircraft Communications Addressing and Reporting System (ACARS). The ACARS air/ground environment is described in ARINC Specification 618. The capabilities of onboard equipment are defined in ARINC Characteristics 597, 724 and 724B. Other standards may also apply. The aviation industry is also currently developing enhanced systems for AOC communications which will provide higher data rates and improved networking protocols than those which are available via ACARS.
A large commercial aircraft typically provides three VHF antennas for ATS and AOC communicationsxe2x80x94typically two of these are dedicated to ATS voice and the third is dedicated to AOC. A partial shift to digital voice is planned for the future in some regions.
The International Civil Aviation Organization (ICAO) has recently recommended for adoption a new radio communications system and protocol known as VHF Data Link Mode 4 (VDL/4). This radio communications system employs a minimum of two frequency channels and can optionally support additional channels.
A single VDL/4 line replaceable unit (LRU) comprises a chassis and associated VDL/4 electronics which support a minimum of two frequency channels and can optionally support additional channels. When a VDL/4 LRU is connected to a single antenna it can typically receive on multiple frequencies at once or transmit on any single frequency. A VDL/4 LRU connected to a single antenna is typically incapable of receiving on any channel during periods when it is transmitting on any single channel, although future advances in technology could enable simultaneous transmission on frequency f1 and reception on frequency f2 if the frequency separation xcex94f=f1xe2x88x92f2 is sufficiently great. A VDL/4 LRU connected to two or more antennas (for example top-mounted and bottom-mounted) may be capable of simultaneous transmission on frequency f1 via one antenna and reception on frequency f2 via another antenna using current technology.
In transport category aircraft, overall operational reliability is typically enhanced by use of dual-redundant and sometimes triple-redundant systems. When applied to packet data communications e.g. VDL/4, operational reliability may be enhanced with a dual installation comprising two VDL/4 LRUs and two antennas, each VDL/4 LRU connected to a single antenna. In this type of installation the two VDL/4 LRUs are typically denoted as a xe2x80x9cleftxe2x80x9d and xe2x80x9crightxe2x80x9d LRU. Failure of the xe2x80x9cleftxe2x80x9d LRU can be compensated by the continued operation of the xe2x80x9crightxe2x80x9d LRU. Individual receiver and transmitter units, contained within the two VDL/4 LRUs, can also be dynamically re-assigned to different channels by a human operator or automatic control system capable of commanding the two VDL/4 LRUs (in some cases switching the receive or transmit function for a given frequency channel from one LRU to another LRU) in order to minimize the total number of receiver units and transmitter units needed to achieve a desired level of operational flexibility and reliability. This type of redundant installation is described in working paper 49 of ICAO/AMCP/7 (Montreal, Mar. 22-30, 2000). Since each VDL/4 LRU is connected to a single antenna in this type of installation, each VDL/4 LRU may be incapable of receiving on any frequency while it is transmitting on any single frequency. If two antennas were provided to each VDL/4 LRU, each VDL/4 LRU would have the potential to receive on frequency f1 while transmitting on frequency f2, but this configuration requires two antennas for each of two VDL/4 LRUs, for a total of four antennas. If two antennas were shared so that each antenna is connected to both VDL/4 LRUs simultaneously, the number of antennas can be limited to two but in this case the signal strength available to each VDL/4 LRU is reduced.
A concern of aeronautical radio communications systems, as for all radio communications systems, is the need to minimize the effects of environmental noise, cosite noise, radio-frequency propagation anomalies and antenna gain effects which can adversely affect communications reliability. In the aeronautical VHF bands, these factors are typically much more significant that receiver-generated noise.
FIG. 1 illustrates a dual-redundant configuration of antennas and radio LRUs known to the prior art. In this configuration a first antenna 11 and first radio LRU 12, and a second antenna 13 and second radio LRU 14, operate in parallel and support human (e.g., pilot and copilot) and other avionics systems 15 operational needs. Two antennas and two radio LRUs exist in this configuration, but the radio LRUs do not cooperate at a peer level. Instead they operate in accordance with commands issued by human operators or other avionics systems. For packet data transmission, only one radio LRU may be used at a time on any single frequency since transmission by two radio LRUs at the same time on the same frequency would result in garbled transmissions (even if the data transmitted by the two radio LRUs is the same). For packet data reception, again one radio LRU may be used with the other reserved as a spare (even if it is active); alternatively both may deliver received data to other onboard systems. In the latter case where both radio LRUs deliver data to other onboard systems, care must be exercised to ensure that the delivery of multiple copies of identical data does not adversely affect onboard operations.
In some cases involving packet data communications, a data link layer technical acknowledgement is required to be sent when a data packet is received. In order to prevent the simultaneous transmission of this technical acknowledgement by radio LRU 12 and radio LRU 14, which could lead to garbled transmissions, the acknowledgement means may be contained within the aggregate of other onboard systems 15 and may be connected to only one of the radio LRUs 12 or 14 at any one time. However, this may create a single point of failure unless redundant acknowledgement means are provided within the aggregate of other onboard systems 15, with appropriate switching mechanisms between the multiple radio LRUs and multiple acknowledgement means.
For some radio systems, e.g. VDL/4, accurate position and time information is used as part of the nominal channel management scheme, and may be provided by a GNSS antenna 16 and GNSS user receiver device 17 or alternative navigation means and an accurate clock. The position and time information may be passed to the radio LRUs and other onboard systems, as required, via direct interwiring from source systems or via other intermediate systems.
This invention is an application of antenna diversity and two or more multi-channel radio LRUs, with a novel cooperative sharing strategy among the multi-channel radio LRUs, to aeronautical packet data communications. Existing avionics and systems are not designed to accommodate antenna diversity, which is considered to increase the overall level of complexity thereby impairing operational reliability and potentially adding to cockpit workload. The present invention overcomes these concerns and offers the following benefits:
a) Enhanced data communications performance in a fading environment;
b) Enhanced data communications performance at long range;
c) Enhanced data communications performance in the presence of cochannel interference;
d) Extendable to an arbitrary number of cooperating peer radio LRUs;
e) No single point of failure;
f) Improved on-aircraft testing of radio functionality;
g) No increase in pilot workload or change in operational procedures.